1. Field of the Invention
The present invention relates generally to a gas laser, for example, He-Ne laser, and is directed more particularly to a gas laser in which a mirror is directly abutted to the end surface of a plasma tube.
2. Description of the Prior Art
In a prior art gas laser, as shown in FIG. 1, there is provided an envelope 1 formed of a cylindrical glass tube. A plasma tube 2, which consists of a pair of glass bore tubes or glass capillary tubes 2a and 2b, is located on the central axis of the envelope 1. Within the envelope 1 and around the plasma tube 2, there is disposed a cylindrical electrode 3 which will serve as one of the electrodes, for example, a cathode, and a terminal 4 is led out from the cylindrical electrode 3 outside the envelope 1. A mirror 5 is attached to the outer end of the glass bore tube 2a of the plasma tube 2 outside the envelope 1, and a Brewster window 6, for example, is attached to an outer end of the other glass tube 2b outside the envelope 1. Another electrode, for example, anode 7 is provided in connection with the glass bore tube 2a such that the anode 7 passes through the glass bore tube 2a into the inside thereof at the outside of the envelope 1.
With the above prior art gas laser, it is necessary that the mirror 5 have the mirror surface perpendicular to the axis of the plasma tube 2. Accordingly, the mirror 5 is attached to the glass bore tube 2a of the plasma tube 2 in such a manner that the mirror surface of the mirror 5 abuts against an outer end surface 8 of the glass bore tube 2a which surface 8 is worked perpendicular to the axis of the plasma tube 2 or glass bore tube 2a and that, for example, glass frit material 9 is used to seal up the connecting portions of the mirror 5 and the glass bore tube 2a of the plasma tube 2.
When the glass frit 9 is used to seal up as set forth just above, a crack is generated in the vicinity of the sealed portion. The reason for this is that stress is applied to the connected portions by the frit, due to the difference between the thermal expansion coefficients of the mirror 5 and the glass bore tube 2a. That is, in general, a multilayer dielectric film 10 is coated on the inner or mirror surface of the mirror 5, so that the mirror 5 is selected sufficiently great in its thickness as to prevent it from being curved upon the coating of the film 10. An optical glass having relatively high thermal expansion coefficient is used as the material of the mirror 5. As a result, the thermal expansion coefficient of the mirror 5 is much different from that of the plasma tube 2. Further, a distortion is caused on the end surface 8 of the glass bore tube 2a upon its working, the frit glass 9 is positioned near the multilayer dielectric film 10 on the inner surface of the mirror 5 and the end surface 8 of the tube 2 a, and so on, so that a crack is apt to be caused at the connecting portion of the plasma tube 2 consisting of the glass bore tubes with the mirror 5 upon sealing the connecting portion by the frit glass 9. Further, there is a defect in that the dielectric film 10 is broken.
Therefore, it may be considered that the sealing of the mirror 5 to the plasma tube 2 is achieved with an organic adhesive agent such as epoxy resin or the like. The organic adhesive agent, however, lacks thermal stability, so that upon evacuating the gas laser, the heating for baking can not be performed sufficiently. Further, since the organic adhesive agent itself produces gas, the gas laser can not be maintained at a constant pressure for a long period of time and cannot maintain the purity of the gas with the result that its shelf life or stack life is shortened.
In order to avoid the above defect, an improved structure is herein disclosed. As shown in FIG. 2, a junction metal tube 11 made of, for example, Kovar (Tradename) is glazed to the outer end of the plasma tube 2, and a deformable tube body 13 having a flange 12 made of, for example, nickel is connected to the outer end of the junction metal tube 11, and a metal tube 14 having a dish-shape end made of, for example, Fe-Ni-Cr alloy is attached to the outer end of the tube body 13, and the mirror 5 with the dielectric film 10 is sealed in the dish-shaped metal tube 14 by the frit glass 9. In this case, the respective thermal expansion coefficients of the metal tubes 11, 13 and 14 are sealed such that the thermal expansion coefficients become successively near to that of the glass of the mirror 5 from that of the glass of the plasma tube 2. The axis alignment of the mirror 5 with the plasma tube 2 is performed by gripping the flange 12 of the metal tube 13 and deforming the same. Accordingly, in this case, it requires that much skillfulness and time are necessary to achieve the alignment of the mirror 5 and the accuracy thereof is relatively low.